1. Field of the Invention
The present invention relates to a flyback converter which is applied to a power supply of a portable computer such as a notebook PC, and more particularly to a flyback converter with a synchronous rectifier which is operated in a critical conduction mode to turn on/off a main switch at a zero crossing point of an output voltage and is adapted to control the duty cycle of a synchronous switch using a Zener diode, resulting in no need for a Schottky diode to be connected in parallel with the synchronous switch and, thus, simplification in circuit design.
2. Description of the Related Art
Recently, electronic and electric appliances have had an increasing number of functions desired by customers, have been increasingly digitized to perform such multiple functions, and have keenly required a small-size, high-efficiency power supply to provide services appropriate to the customers' various desires.
Among existing power supplies, a flyback power supply is most widely used owing to the advantage of having a smaller number of components enabling lower-cost production and miniaturization.
However, recently, electronic and electric appliances have required a lower-voltage, larger-current power supply, so that a diode rectifier system for an existing flyback converter cannot meet smallness, thinness and high-efficiency conditions desired by the customers any longer. Furthermore, since loss occurs in proportion to output current, the diode rectifier system may suffer excessive loss if the output current is large.
In this connection, a synchronous rectifier system using a semiconductor switch with small conduction loss, such as a MOS (Metal-Oxide Semiconductor) transistor, has been proposed to replace the diode rectifier system.
FIG. 1 is a circuit diagram illustrating the concept of a general flyback converter, and FIG. 2 is a timing diagram of main signals in FIG. 1.
In FIGS. 1 and 2, an alternating current (AC) input voltage Vin is rectified by a rectifier 11 and then provided to a transformer TF. At this time, a flyback switching circuit 12 switches a main switch MS, so that the main switch MS is turned on/off repeatedly as shown in FIG. 2.
A drain-source voltage Vds1 of the main switch MS varies with the on/off operation of the main switch MS, as shown in FIG. 2.
That is, if the main switch MS is turned on, primary current I1 flows in a primary coil L1 of the transformer TF through the main switch MS, as shown in FIG. 2, and, at the same time, a synchronous switch SS is turned off by a synchronous switching circuit 14. At the time that the main switch MS is turned off, the synchronous switch SS is turned on by the synchronous switching circuit 14, so that energy in the primary coil L1 of the transformer TF is induced to a secondary main coil L21 of the transformer TF, thereby causing secondary current I2 to flow as shown in FIG. 2.
Here, Vds1 is a voltage applied across the main switch MS, and Vds2 is a voltage applied across the synchronous switch SS.
The drain-source voltage Vds2 of the synchronous switch SS varies with the on/off operation of the synchronous switch SS, as shown in FIG. 2. Here, the synchronous switching circuit 14 is supplied with a driving voltage from a secondary auxiliary coil L22 of the transformer TF connected to the secondary main coil L21 thereof.
Through this process, a voltage in the secondary main coil L21 of the transformer TF is supplied as an output voltage Vout via an output capacitor Co.
As can be seen from the above description, the synchronous switching circuit 14 for the synchronous switch SS is turned on/off synchronously with the flyback switching circuit 12, so the synchronous switch SS, which is a MOS transistor, acts as a rectifier.
This flyback converter can be classified into various types according to technical details, such as driving methods for the main switch and synchronous switch and the design of a driving circuit for the synchronous switch, and one example of flyback converters of such various types will hereinafter be described with reference to FIGS. 3 and 4.
FIG. 3 is a circuit diagram of a conventional flyback converter.
With reference to FIG. 3, the conventional flyback converter comprises a voltage source, a flyback switching circuit 20 for receiving a voltage from the voltage source and outputting a high-frequency pulse to a switch S1, a transformer T1 having a primary coil connected to the flyback switching circuit 20 for receiving the high-frequency pulse therefrom, and two secondary coils acting as a master source and a sub-source, respectively, a synchronous rectifier 30 connected to a current sensor 40 and the sub-source for outputting a driving pulse, and a synchronous switch M1 connected in parallel with an output diode D2 for receiving the driving pulse from the synchronous rectifier 30. The output diode D2 has one end connected to the master source and the other end connected to a load 50, which is in turn connected in parallel with an output capacitor. The current sensor 40 is connected in series to the load 50 to detect load current. The current sensor 40 also acts to transfer the detected load current to the synchronous rectifier 30. Here, V1 denotes a power source, 21 denotes a feedback circuit, and 25 denotes a gate control circuit.
Details of the flyback converter of FIG. 3 are disclosed in U.S. Pat. No. 6,353,544.
FIGS. 4a and 4b are waveform diagrams of currents in respective operation modes of the flyback converter of FIG. 3.
With reference to FIGS. 3 to 4b, in the conventional flyback converter, the switch S1 is operated at a fixed frequency, so that it is appropriate to a discontinuous conduction mode (DCM) of FIG. 4a and a continuous conduction mode (CCM) of FIG. 4b. 
FIG. 5 is a waveform diagram of reverse recovery current in the flyback converter of FIG. 3.
In the conventional flyback converter of FIG. 3, in the CCM, which is a main operation mode of the flyback converter, while a large amount of secondary current I2 flows, the synchronous switch M1 is turned off at the moment that the switch S1 is turned on. In this case, reverse recovery current (RRC) may be instantaneously generated due to PN junction characteristics of the synchronous switch M1. In order to prevent such RRC from being generated, the output diode D2, which is a Schottky diode, is connected in parallel with the synchronous switch M1. That is, the generation of RRC is prevented by a Schottky diode with little reverse recovery time.
However, the aforementioned conventional flyback converter is disadvantageous in that the synchronous switch driving circuit is complicated because the secondary auxiliary coil is necessary for supply of a driving voltage to the synchronous switch and the flyback converter is operated in both the DCM and CCM. Moreover, in order to prevent the generation of RRC, the Schottky diode must be connected in parallel with the synchronous switch, causing reduction in available area of a printed circuit board (PCB) and, in turn, increase in production cost.